Re: [Xen-devel] blkif migration problem

[Ewan Mellor <ewan@xxxxxxxxxxxxx>]

On Thu, Dec 07, 2006 at 03:47:39PM +0000, Cristian Zamfir wrote:

> 
> Hi,
> 
> I am trying to live migrate blkif devices backed by drbd devices and I 
> am struggling with a problem for a few days now. The problem is that 
> after migration, the domU machine cannot load any new programs into 
> memory. The ssh connection survives migration and I can run programs 
> that are already in the memory but not something that needs to be loaded 
> from the disk.
> 
> I am currently testing with an almost idle machine and I am triggering 
> the drive migration after the domain is suspended, in step 2, from: 
> XendCheckpoint.py: dominfo.migrateDevices(network, dst, 
> DEV_MIGRATE_STEP2, domain_name).
> 
> However, I also tried before the domain is suspended from step 1 
> (dominfo.migrateDevices(network, dst, DEV_MIGRATE_STEP1, domain_name))
> and everything works fine, except that there is the obvious possibility 
> of loosing some writes to the disk because the domain is not suspended yet.
> 
> After migration, when I reattach a console I get this message:
> "vbd vbd-769: 16 Device in use; refusing to close"
> This is from the blkfront.c backend_changed() function but I cannot 
> figure out why this error occurs.

I believe that this means that the frontend has seen that the backend is
tearing down, but since the device is still mounted inside the guest, it's
refusing.  I don't think that the frontend ought to see the backend tear down
at all -- the guest ought to be suspended before you tear down the backend
device.

When you say that you are "triggering the drive migration", what does that
involve?  Why would the frontend see the store contents change at all at this
point?

Have you tried a localhost migration?  This would be easier, because you don't
actually need to move the disk of course, so you can get half your signalling
tested before moving on to the harder problem.

Ewan.

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